Technological development and increased demand for mobile equipment have lead to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, a great deal of research and study has been focused on a lithium secondary battery having high energy density and discharge voltage and thus some of such lithium secondary batteries are commercially available and widely used. The lithium secondary battery generally uses a lithium transition metal oxide as a cathode active material and a carbonaceous material as an anode active material.
However, the anode based on the carbonaceous material has a maximum theoretical capacity of only 372 mAh/g (844 mAh/cc), thus suffering from limited increase of capacity thereof. Lithium metals, studied for use as the anode material, have a high energy density and thus may realize high capacity, but raise problems associated with safety concerns due to growth of dendrites and a shortened charge/discharge cycle life as the battery is repeatedly charged/discharged.
For these disadvantages and problems, a number of studies and suggestions have been proposed as to silicon, tin or alloys thereof, as a possible candidate material exhibiting high capacity and being capable of substituting for the lithium metal. For example, silicon (Si) reversibly absorbs (intercalates) and desorbs (deintercalates) lithium ions through the reaction between silicon and lithium, and has a maximum theoretical capacity of about 4200 mAh/g (9366 mAh/cc, a specific gravity of 2.23) that is substantially greater than the carbonaceous materials and thereby is promising as a high-capacity anode material.
However, upon performing charge/discharge processes, silicon, tin or alloys thereof react with lithium, thus undergoing significant changes of volume, i.e., ranging from 200 to 300%, and therefore continuous charge/discharge may result in separation of the anode active material from the current collector, or significant physicochemical changes in contact interfaces between anode active materials, which is accompanied by increased resistance. Therefore, as charge/discharge cycles are repeated, the battery capacity sharply drops, thus suffering from a shortened cycle life thereof. For these reasons, when the binder for a carbon-based anode active material, without any special treatment or processing, is directly applied to a silicon-based anode active material, it is impossible to achieve desired effects.
In order to cope with such problems, a certain prior art has proposed a method for inhibiting lowering of binding force between the current collector and anode active material and/or between anode active materials, resulting from volume changes of the silicon-based anode active material, which uses polyamide acid as a binder and involves applying an anode mix including polyamide acid as the binder to the current collector and heat-treating the applied anode mix at a high temperature (higher than 300° C.), thereby converting polyamide acid into polyimide via imidation. However, this method requires heat-treatment at a high temperature for a prolonged period of time (for example, 10 hours) and thus presents problems associated with remarkably lowered productivity of the battery.
With respect to such problems, the present invention suggests a technique using a material having photo-polymerizable properties as the binder. Utilization of a photo-polymerizable material for fabricating electrodes in secondary batteries is known in some of secondary batteries employing polymer electrolytes.
For example, Japanese Patent Laid-open Publication No. Hei 10-162832 discloses a technique of manufacturing a cathode or anode via photo-polymerization, using a monomer having polymerizable functional groups at ends of the molecular chain and containing a fluoro-alkyl group in the molecular structure thereof, as a binder.
In addition, Japanese Patent Laid-open Publication No. 2005-44681 discloses a technique of manufacturing a cathode or anode via photo-polymerization or thermal polymerization, using an urethane (meth)acrylate-based compound (A) having (meth)acryloyl groups at ends of the molecular chain thereof and/or a polyisocyanate-based derivative (B) where at least one of the molecular chain ends is a (meth)acryloyl group and the remainder is a hydrocarbon group, as a binder.
Such binders adopted in the above-mentioned Japanese Patents are binders for fabricating the cathode or anode in a lithium secondary battery which is primarily based on a polymer electrolyte and may offer superior cycle characteristics to conventional fluorine-based polymers such as polyvinylidene fluoride (PVDF). However, it was confirmed that these binders did not provide a desired level of cycle characteristics in secondary batteries using silicon- or tin-based anode active materials undergoing much further volume changes during a charge/discharge process, as compared to carbon-based anode active materials.
As such, there is an urgent need for the development of battery manufacturing technology which provides strong binding force sufficient to inhibit significant volume changes of anode active materials occurring during a charge/discharge process in lithium secondary batteries using silicon- or tin-based anode active materials and is also economical in terms of a manufacturing process.
Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have surprisingly discovered that, in a lithium secondary battery using silicon- or tin-based anode active materials, use of a certain photo-polymerizable material as a binder for an anode mix leads to significantly improved charge/discharge characteristics of the battery via a low increase of resistance resulting from less occurrence of interfacial changes between active materials, in spite of significant volume changes in the anode active material occurring upon charging/discharging the battery, and high binding force between active material and current collector, thus inhibiting easy separation therebetween. The present invention has been completed based on these findings.